Structural Dynamics of Electronic and Photonic Systems

Ephraim Suhir, T. X. Yu, David S. Steinberg, Ephraim Suhir, T. X. Yu, David S. Steinberg

• Publisher: Wiley
• Year: 2011
• Print ISBN: 9780470250020
• eBook ISBN: 9780470886793
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Technology & Engineering Research & Skills
• Index: Technology & Engineering

The proposed book will offer comprehensive and versatile methodologies and recommendations on how to determine dynamic characteristics of typical micro- and opto-electronic structural elements (printed circuit boards, solder joints, heavy devices, etc.) and how to design a viable and reliable structure that would be able to withstand high-level dynamic loading.Particular attention will be given to portable devices and systems designed for operation in harsh environments (such as automotive, aerospace, military, etc.) In-depth discussion from a mechanical engineer's viewpoint will be conducted to the key components' level as well as the whole device level.Both theoretical (analytical and computer-aided) and experimental methods of analysis will be addressed. The authors will identify how the failure control parameters (e.g. displacement, strain and stress) of the vulnerable components may be affected by the external vibration or shock loading, as well as by the internal parameters of the infrastructure of the device. Guidelines for material selection, effective protection and test methods will be developed for engineering practice.

Chapter 1

Some Major Structural Dynamics-Related Failure Modes and Mechanisms in Micro- and Opto-Electronic Systems and Dynamic Stability of These Systems

David S. Steinberg

Steinbergelectronics, Inc.

Westlake Village, California

1 Physics of Electronic Failures in Vibration and Shock

Modern electronic equipment is being used in a very large number of different areas that range from simple applications, such as automobile keys and temperature control devices, to very complex applications, such as airplanes, space exploration vehicles, and optical scanning medical devices. It is probably safe to say that virtually all electronic systems will be exposed to some form of vibration or shock during their lifetime.

The vibration and shock exposure may be due to the operating environment experienced by an airplane or an automobile. The vibration and shock exposure may also be due to shipping the product across the country by truck or train. Electronic systems that are required to operate in a harsh shock or vibration environment will often fail. If a failure occurs in an automobile temperature-sensing device or a fuel gage, it may be inconvenient for the owner, but the chances of someone being injured or killed are small. If the electronic failure happens to occur in the flight control system or navigation control system of an airplane or a missile, several hundred people could be injured or killed.

Many different types of materials, for example, metals, ceramics, plastics, glass, and adhesives, are being used today to fabricate and assemble a wide variety of electronic systems for commercial, industrial, and military applications. Many types of sophisticated electronic component parts are now available from a large number of manufacturers for specific applications and functions that did not exist only a decade ago. These components are often soldered to multilayer printed circuit boards (PCBs). PCBs may have from 6 to 12 internal layers of thin copper ground planes and voltage planes to remove heat and to provide electrical interconnections. The PCBs make it easy to assemble, remove, and maintain complex sophisticated electronic equipment at reduced costs. The PCBs must also protect the electronic components in storage, shipping, and operation in severe vibration, shock, thermal, and high-humidity environments. A wide variety of special plastics and metals are available to fabricate cost-effective and reliable electronic systems for special conditions.

The soldering process must be carefully controlled because solder is often the major source of early failures in the field. Good solder joints usually require the use of paste and flux to obtain a reliable connection. The paste and flux must be carefully removed from the PCB to prevent electrical malfunctions in sensitive systems after several months of operation in harsh environments. The normal procedure is to mount the electronic components slightly above the surface of the PCB, so that there is a small gap under the components. This makes it easy to flush out any paste and flux accumulated under the components. A thin protective coating should be applied to the PCB after cleaning to avoid the growth of dendrites, which can degrade the electrical performance of sensitive electronic systems operating in humid conditions. Dendrites are thin semitransparent plastic whiskers with a high electrical impedance that will often grow between electrical conductors in the presence of a chemical residue such as paste and flux and moisture exposed to an electrical current. Extended exposure for periods of several months can produce such a large mass of these whiskers that it will change their electrical resistance to an extent sufficient to cause malfunctions and even short circuits in the electrical system. Several thin protective coatings are available that can effectively prevent the growth of dendrites on PCBs. Materials such as paralyne, solder mask, polyurethane, epoxy, and acrylics can be applied to the clean PCB surfaces using different methods such as spray, brush, dip, and even vacuum processes. One of the best materials for protection is paralyne, which can be applied as a vapor. However, it is expensive and very tough. It is difficult to remove from the PCB if repairs have to be made and can often create new failures while trying to repair the old failures.

PCBs are often enclosed within a box or housing for easy transportation and handling. The housing can also protect sensitive electronic components from hostile external environments, such as sand, dust, sun, humidity, rain, insects, mice, and birds. Very small insects often make nests inside the warm interior of the electrically operating system. Their residue can build up inside the housing and cause bridging across multiple pin connectors, resulting in short circuits with early electrical failures. Electronic systems that will be required to operate in open outside areas must be fabricated and assembled to prevent small insects from entering the housing and making nests. Removable covers must have a very close fit or gaskets must be used to provide a good seal.

The PCBs are normally attached to the inside walls of the housing to help conduct away excessive internal heat to the outside ambient, where it is carried away. This also prevents the PCBs from impacting against each other and causing damage in vibration and shock conditions or the PCBs may have a multiple pin or socket connector added to one end of each PCB with a mate on the housing, so that each PCB can be plugged into the housing for electrical operation. Side wedge clamps can be used on the PCBs or the housing to improve the internal conduction heat flow path to the outside of the housing to reduce internal hot-spot temperatures. Reducing the internal hot-spot temperatures will usually increase the fatigue life of the electronics. The use of wedge clamps also helps to support the sides of the PCBs, which increases the PCB stiffness and natural frequency.

A higher PCB natural frequency substantially reduces the PCB dynamic displacements in vibration and shock conditions. This reduces the stresses in the PCBs, in the components, in external lead wires, and in external solder joints. This increases their fatigue life. Reducing the PCB dynamic displacements also increases the fatigue life of the die bond wires and the ball bonds inside the electronic components. Therefore, by increasing the PCB natural frequency, one could increase substantially the fatigue life of the lead wires, the solder joints, and the ball bonds on the semiconductor components mounted on the PCBs.

The natural (or resonant) frequency of the outer housing must be well separated from the natural frequency of the internal PCBs to avoid severe dynamic coupling and rapid structural failures of the PCBs in the housing during sine vibration. When the natural frequency of the outer housing is excited during exposure to a sine wave, the housing can sharply amplify the magnitude of the input acceleration (g) level, depending upon the damping in the system. As is known from the theory of damping in linear vibration systems, when the structural damping in the system is zero and the system is being vibrated at its natural frequency using a sine wave, the transmissibility (amplitudes) of that structure will be infinite. This condition is impossible, however, because every real structural system has damping.

2 Case History for Design, Analysis, and Testing of Electronic Chassis Required to Operate in Severe Sine Vibration Environment and Effects of Using Viscoelastic Damping Material on PCBs to Increase Fatigue Life

Failures in electronic systems can occur in many ways, often due to carelessness and lack of experience in handling a new environment or a new material. A large company with extensive electronics experience was awarded a multi-million-dollar contract for a program with a very severe vibration environment. Several other companies no-bid this contract because of the potential problems with the environment and low weight requirements for a system about the size of a shoe box. The company put a team of its top engineers together to solve the problem. This team spent several months investigating different proposals involving different exotic materials and the use of prototype test models to prove their capability to withstand the severe environment. The standard approach for providing reliable operation in severe vibration is to use vibration isolators. However, in this case the equipment had to be hard mounted so that isolators would not work. The team of experts finally selected a fabrication method for the PCBs inside the electronic enclosure that used a viscoelastic material. When this material was bonded to the PCBs, it provided high damping. The prototype vibration test models showed excellent results. The viscoelastic material was very effective in reducing the vibration “G response” levels enough to assure reliable operation in the severe vibration conditions. Reports were written and presentations were made to upper management. The experts' team was given a green light to proceed with the fabrication and assembly for a large number of production units. Everyone was happy and sure that they had a reliable lightweight design that would survive the required severe qualification vibration tests. Their successful prototype test data were proof their new design would not fail. Their customer was invited to witness the qualification tests. The day for the qualification tests arrived. One of the customers selected one chassis assembly to start the vibration tests. The selection was made from a group of about 40 production units, ready for delivery to the customer. The first part of the vibration sine sweep tests went well, with no problems, so that everyone was happy. The next phase was the nasty 60-min dwell at the primary resonant frequency of the chassis. It was a mess. There were very loud cracking noises and a complete electrical failure of the electronic unit after a few minutes into the resonant dwell. The chassis was removed and the top cover was opened to inspect the condition of the PCBs inside of the chassis. Dozens of electronic components that had been soldered to the PCBs had broken off and were now lying at the bottom of the chassis. It was a major disaster. It was obvious that the design of the chassis did not meet the contract environment requirements. It was also obvious that the other 39 chassis assemblies sitting on the shelf ready to be delivered to the customer were also unacceptable.

An investigation of the failures showed that all the prototype models were tested at room temperature. No one in the expert team had any experience with the properties of viscoelastic damping materials. All the data they had from various sources showed the general material had excellent damping properties. No one in the expert team thought of calling or talking to the various viscoelastic suppliers to get more information on these materials for their severe environments. After all, the expert group had their own test data that showed the viscoelastic material was acceptable for their environment. What else did they have to know? What they did not know was that most viscoelastic materials are extremely sensitive to temperatures. At room temperatures and lower, these materials work very well and can provide good damping. However, at higher temperatures these materials can lose almost all their damping properties and are almost useless. Since all the prototype fabricated test models were tested at room temperatures, the loss of damping at higher temperatures was not observed. No one in the expert group thought of the heat that can be generated when the electronic system was in operation. All the existing finished assembled units ready for delivery were now scrap, a whole new system had to be developed and put into production as quickly as possible, and the new system would still have to survive the severe environment. The company had a contract to deliver a large number of systems at a predetermined price. They had two choices. Go bankrupt or go ahead with a new design and production to meet the contract with their cost of about $25 million. They chose to redesign the unit and to meet the contract with their own money.

The company searched for other engineers that had the knowledge and experience to meet the contract requirements. The new engineers were asked if they had ever used viscoelastic damping to improve the reliability of electronics in severe vibration. Their answer was that their experience showed that damping does not work very well, so they prefer not to use those techniques. Instead, the new engineers recommended the use of snubbers to reduce the dynamic displacements of the PCBs in severe vibration and shock conditions. Snubbers are usually made from epoxy fiberglass dowel rods, 0.25 in. in diameter. When several plug-in types of PCBs of the same size are inserted in parallel groups, the dowel rods should be epoxy bonded at the center of every PCB on both sides.

The dowel rods on adjacent parallel PCBs should be facing each other with a small gap about 0.005–0.010 in. between facing dowel rods. These snubbers can reduce the dynamic displacements of the PCBs in severe vibration and shock conditions. This then reduces the forces and stresses acting on the electronic components mounted on the PCBs, which substantially increases the fatigue life of the equipment (see [1], Fig. 7.7, p. 160). Snubbers are successfully used in PCBs for air dropping electronic sensors in the ocean at an altitude of 1000 ft. Shock levels of 1000g are produced under these conditions with very few failures.

3 What Happens When 20 Plug-in PCBs Are Tied Together, Then Installed in a Chassis That Is Subjected to a 5G Peak Sine Vibration Input Level?

Another case history that demonstrates types of electronic fa...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Contributors
• Chapter 1: Some Major Structural Dynamics-Related Failure Modes and Mechanisms in Micro- and Opto-Electronic Systems and Dynamic Stability of These Systems
• Chapter 2: Linear Response to Shocks and Vibrations
• Chapter 3: Linear and Nonlinear Vibrations Caused by Periodic Impulses
• Chapter 4: Random Vibrations of Structural Elements in Electronic and Photonic Systems
• Chapter 5: Natural Frequencies and Failure Mechanisms of Electronic and Photonic Structures Subjected to Sinusoidal or Random Vibrations
• Chapter 6: Drop/Impact of Typical Portable Electronic Devices: Experimentation and Modeling
• Chapter 7: Shock Test Methods and Test Standards for Portable Electronic Devices
• Chapter 8: Dynamic Response of Solder Joint Interconnections to Vibration and Shock
• Chapter 9: Test Equipment, Test Methods, Test Fixtures, and Test Sensors for Evaluating Electronic Equipment
• Chapter 10: Correlation between Package-Level High-Speed Solder Ball Shear/Pull and Board-Level Mechanical Drop Tests with Brittle Fracture Failure Mode, Strength, and Energy
• Chapter 11: Dynamic Mechanical Properties and Microstructural Studies of Lead-Free Solders in Electronic Packaging
• Chapter 12: Fatigue Damage Evaluation for Microelectronic Components Subjected to Vibration
• Chapter 13: Vibration Considerations for Sensitive Research and Production Facilities
• Chapter 14: Applications of Finite Element Analysis: Attributes and Challenges
• Chapter 15: Shock Simulation of Drop Test of Hard Disk Drives
• Chapter 16: Shock Protection of Portable Electronic Devices Using a “Cushion” of an Array of Wires (AOW)
• Chapter 17: Board-Level Reliability of Lead-Free Solder under Mechanical Shock and Vibration Loads
• Chapter 18: Dynamic Response of PCB Structures to Shock Loading in Reliability Tests
• Chapter 19: Linear Response of Single-Degree-of-Freedom System to Impact Load: Could Shock Tests Adequately Mimic Drop Test Conditions?
• Chapter 20: Shock Isolation of Micromachined Device for High-g Applications
• Chapter 21: Reliability Assessment of Microelectronics Packages Using Dynamic Testing Methods
• Chapter 22: Thermal Cycle and Vibration/Drop Reliability of Area Array Package Assemblies
• Chapter 23: Could an Impact Load of Finite Duration Be Substituted with an Instantaneous Impulse?
• Index